This invention relates to a method and apparatus for time division multiplexing digital signals, and more particularly to a frame alignment technique for multiplexing plural digital signals.
There is known a so-called floating frame method of time division multiplexing plural tributary digital signals into higher bit rate digital signals. According to such time division multiplexing method as discussed in U.S. Pat. No. 4,698,806 and in American National Standard, TIX 1,4/87-505R2, 1987 at page 13, higher end bit rate transmission frames are handled asynchronously to thus realize reduction of delays and buffer capacities during frame alignment.
As shown in FIG. 2, the head portions indicated by a letter F of lower bit rate signals A1, A2 and A3 multiplexed into a higher bit rate transmission frame A are not aligned, i.e., the lower bit rate frame phases are asynchronous with each other, when adopting the floating frame method. Similarly, the lower bit rate frame phases of signals B1, B2 and B3 multiplexed into another higher bit rate transmission frame B are not aligned.
Lower bit rate signals A2, B1 and B3, by way of example, cross-connected from such higher bit rate signals and multiplexed into a signal C are also asynchronous with each other. Frame alignment is adapted to be effected only when lower bit rate signals are inputted to an apparatus such as a switching apparatus requiring frame phase synchronization, so that a delay to be caused by the frame alignment is suppressed to the minimum since digital signal processing in the signal path is carried out without frame alignment.
The phase of a higher bit rate transmission frame is used as a reference phase for frame alignment of respective lower bit rate signals. More particularly, a higher bit rate signal takes a frame configuration as shown in FIG. 3, and a phase difference between the higher bit rate transmission frame and the head position of each lower bit rate signal is identified by the value of each corresponding pointer by which frame alignment can be conducted. The pointer is derived from each lower bit rate signal on the basis of a frame synchronizing signal of the higher bit rate transmission frame. Each element shown in FIG. 3 corresponds to a segment constituting each frame, the segment being constructed of, e.g., 1 byte of data. An arrow indicates the order of data transmission, namely, the higher bit rate transmission frame together with its frame overhead OH shown in FIG. 2 is transmitted in a time division multiplexed byte-interleaved manner. In FIG. 3, a set of overhead bytes and a set of respective lower bit rate signal bytes, shown one below another in FIG. 3, are called a column. A frame pattern as depicted in FIG. 3 is a pattern indicating that the location thereof is the head of a 125-microsecond data frame, for example. Pointers P1 to P3 as depicted in FIG. 3 indicate the positions of respective lower bit rate signals relative to the higher bit rate frame.
If lower bit rate signals A2, B1 and B3, for example, are selected from the difference higher bit rate signals A and B shown in FIG. 2 to multiplex them into a new higher bit rate signal C, then it becomes necessary to synchronize the frames of the different higher bit rate signals A and B. In this case, it is not necessary to control the phases of the higher bit rate signals throughout the whole frame length, but only the overhead portions of the higher bit rate signals are controlled and the values of the pointers are accordingly changed by the amount of the shifted phases. However, the phase of each column including pointer P1, P2 or P3 in the frame configuration shown in FIG. 3 becomes different after multiplexing. This is illustrated in FIGS. 4A and 4B. In FIGS. 4A and 4B, it is assumed by way of example and for clarity that the higher bit rate signal A before multiplexing is synchronous with the system phase, and that the higher bit rate signal B is asynchronous with the system phase. For synchronization purposes, OH and B3 of the signal B are stored in a memory and given delays under control of read timings to thus allow the frame alignment or synchronization between the signals A and B. Further, if the phases of lower bit rate signals A2 and B1 to be multiplexed become the same as shown in FIG. 4B, then it becomes necessary to delay one of them relative to the other. The above-described prior art technique does not consider the phase control between lower bit rate signals having a phase collision so that a large capacity and high speed memory, and complicated control have been needed for practical multiplexing operation.